The present invention relates to a control method for a mechanohydraulic system with a degree of freedom for each hydraulic actuator functioning as a controlled element and a device for implementing the method.
Mechanohydraulic systems with a (mechanical) degree of freedom, that is to say systems in which, for example, a mechanical part with a degree of freedom (load system) is actuated via a hydraulic cylinder (actuator), occur, in practice, in the most diverse possible configurations, such as, for example, as a cage roller of a winch, as a loop lifter between two stands of a mill train or as a hydraulic adjuster of a stand of a mill train, but also in general applications, such as positioning tables, vibrating tables, etc. What is common to these systems is that they are basically oscillatable on account of the hydraulic oil column in the hydraulic cylinder or in other resilient elements in the load system. Mention may be made here, as representative examples in no way restricting the general validity, of applications in which, for example, a hydraulic linear cylinder moves a rotatably mounted mass, for example a cage roller, loop lifter, etc. In systems of this type, a pronounced oscillation behavior is exhibited due to the hydraulic oil column which acts in the same way as a spring. This is reflected in an undesirable tendency of the overall system to oscillate at specific points in the frequency response. The resonant frequencies occurring in this case are determined essentially by the equivalent mass of the mechanical system, the geometric conditions and the equivalent spring rigidity of the elasticities occurring, such as, for example, the compressibility of the oil column, and/or the elasticity of a roll stand, etc. It is typical, then, of such systems with pronounced resonant frequencies that they are inclined to (damped) oscillations in the event of regulating actions from outside. In control operations which are aimed, for example, at moving to a new operating point, or the leveling out of a fault introduced from outside, these oscillations in the control operations give rise to extremely undesirable transient variations of physical variables. In the abovementioned example of loop lifters, this has the effect of strip tension fluctuations which lead, in turn, to undesirable contractions of the strip. Where cage rollers are concerned, these fluctuations of the pressure of the cage roller on the strip may lead to surface damage caused by indentations.
In current practice, therefore, controllers are often set only very slowly, in order to keep the excitations of these undesirable oscillations as low as possible. One possibility, known from standard literature, is to use what are known as “notch filters”, narrow-band band-rejection filters, which are aimed at avoiding the excitation of oscillations due to the controller by the directed “tuning out” of the frequency range around the resonant frequency of the system to be controlled, in terms of the controlled variable. A serious disadvantage of this method, especially in the applications mentioned, is that the characteristic of the mechanical system remains unchanged and, even though the controller itself avoids an excitation of oscillations, undetectable faults acting from outside give rise, as before, to oscillations of the system. Also, the resonant frequencies are dependent on the selected operating point.
What has a more serious effect in such systems, however, is that they generally have, as mentioned, a nonlinear behavior. The known methods, such as the use of notch filters, are methods of linear control technology and, in the case of nonlinear systems, have validity only in the vicinity of the operating point, for which the nonlinear element has been approximated by a linear system. It is immediately clear, however, for example in hydraulic linear drives, that, with a variation in the position of the piston of the hydraulic drive and consequently of the oil column, the resonant frequency also changes. In the method described above, there is the possibility of selecting a very broad notch filter, which, in turn, considerably restricts the dynamics of the overall system.